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16 SMT Magazine • June 2016 by Stephen Las Marias I-CONNECT007 In an interview with SMT Magazine, Watson Tseng, General Manager of Shenmao America Inc., talks about the challenges in solder paste printing, key parameters and best practices to consider, and variables involved when it comes to selecting the appropriate solder paste for the printing process. Stephen Las Marias: What are the biggest chal- lenges that your customers face when it comes to solder paste printing? Watson Tseng: Many of our customers are fac- ing greater challenges assembling smaller and smaller components onto their PCBs and flex circuits. Due to the decreased size, the stencil aperture has to be much smaller than before, for example 0.25 mm or less. It increases the challenges of solder paste printing—the area ra- tio (AR) becomes smaller and the transfer rate goes down. Getting a good deposition of solder paste becomes more and more difficult. Some customers increase the printing speed to get higher output rate. Some are printing at 120 or 150 mm/s or even faster. A good solder paste de- signed for high-speed printing is necessary for this application. Otherwise, there will be a lot of shorts or slump failures found after stencil printing. Las Marias: From your perspective, how do the tighter tolerances and even narrower PCB lines and spacing impact the solder paste printing process? Tseng: Yes, they do. Especially when you have both large and small components on your board. It's a huge challenge to do stencil print- ing for this kind of build. A good stencil design, a good solder paste, a precise stencil printer is required to achieve the best result. Las Marias: What about the solder pastes? What are the variables involved when it comes to select- ing the appropriate solder paste for the printing process? SOLDER PASTE PRINTING: Challenges and Best Practices FEATURE INTERVIEW